The following correction was printed in the Guardian's Corrections and clarifications column, Thursday March 16 2006

In the article below, we said that the Medical Research Council funded the play Every Breath about animal rights. That was incorrect. We should have said the Wellcome Trust.

"I heard a protestor say he'd rather kill a researcher than an animal," said a girl in a neat school uniform. "Animal testing is murder. It's just wrong," said a second. "Can't they breed people to be tested on?" suggested another, before descending into a fit of giggles.

This is a snapshot of the rowdy and combative discussion that followed the premiere of a play on animal testing last Monday at Waverley School in Peckham Rye, south London. Every Breath, by Judith Johnson, explores vivisection through the characters in a dysfunctional family. The play, which will be watched by 15,000 schoolchildren across the country and theatregoers at the Edinburgh festival, is aimed at taking the animal rights debate out of the hands of the extremists.

"The extremism is what gets picked up on all the time," says Dr Sophie Petit-Zeman at the Association of Medical Research Charities, which part-funded the project. She says Johnson, who has written for the TV series Grange Hill, was keen to avoid the issue of violence altogether.

The play was guided from the start by a panel representing all sides of the debate, including scientists, animal rights campaigners and a philosopher. All, including the Medical Research Council, which also funded the project, were happy with the final script. "It doesn't reach any conclusions, but it does set out the arguments in quite a detailed way," says Petit-Zeman.

In the play, a university is building a new facility for animal research. Sonny is an 18-year-old vegetarian campaigning peacefully to stop the lab being built. His older sister, Anita, is a hard-headed scientist. As the plot develops, we learn that she is about to embark on a PhD involving rat experiments in the lab Sonny wants closed.

It was a brave decision to eschew the dramatic possibilities that the more extreme end of the animal rights movement would have provided. But that choice stopped post-show discussions veering away from the core question: is it right to put the lives of our family and friends above those of animals?

Sander Van Kasteren, who was on the steering panel, is a PhD student who uses rats in research into techniques for diagnosing multiple sclerosis. "As soon as you engage the animal rights movement in dialogue, you start getting death threats," he says, "By focusing on the mainstream, it allows a real discussion to develop."

The animal rights campaigners on the panel are also pleased with the decision not to focus on violence. "That's not the important issue," says Alistair Currie, of the British Union for the Abolition of Vivisection. "It's a very small minority of people who behave that way."

The play's sibling feud is set alongside a love story between a single mum, Lina, and her unsuitable boyfriend, Raz. He provides the comic relief that stops the play feeling like a lesson. Lina and Raz's first bungled kiss had the 14-year-olds in hysterics and his choice of T-shirt when Lina first brings him home - "Too much sex makes your eyes go fuzzy" - had them rolling in the aisles. "Without those light-hearted bits, the whole play could get quite bogged down," says Darren Saul, who plays Raz.

But it is Anita and Sonny who allow students to connect with the big ideas. "The issues are discussed through the relationships between the characters, so you are learning without realising it," says Katie Donnison, who plays Anita. "To get the kids responding to something emotionally is not a bad thing at all."

Sonny nearly dies from an asthma attack. He has stopped taking his medication because it was tested on animals, but comes to realise he is worth more to the cause alive than dead.

It is Sonny who naturally commands most sympathy. He is passionate and likeable, while his sister comes across as arrogant and cold. Her research is not focused directly on any cure, so her rats die purely in a quest for knowledge. On the face of it, the script doesn't favour the experimentation case.

To my surprise though, it was Anita's arguments that won the kids over in the 40-minute discussion that followed the play. The proportion in favour of animal testing rose from around 30% to 50%.

"You sympathise with him, but he has to have the medication," says Paula Ledger, head of humanities at the all-girls comprehensive. "The girls are thinking, 'if it was my brother, what would I do, what would I want?' I think that's what swayed them really."

Was it difficult to make the play both an engaging story and a source of information? Yes, confesses the director, Nigel Townsend. "I think the problem when we started was that we had too much information. The scientists we were working with felt that, if only we got all this information out, people would agree with them. Which is rubbish." The company's aim, he says, is to present the shades of grey in the argument and leave the students wanting to find out more.

"It gently stimulates the discussion, rather than layering it all on," says the company manager, Thom Hammond, who led the debate after the play.

The students responded with howls of laughter and shrieks of delight and the discussion at the end was full of passion and intelligence. Waverley's catchment covers some of the poorest London neighbourhoods. The play made sense to them.

· Teachers who want to use Every Breath as the launch pad for lessons can get study packs from Y Touring (www.ytouring.org.uk)